[Freeswitch-users] DTMF 2833 not working when switching to dynamic payload type 96

John Narduchi johnnarduchi at gmail.com
Thu Jun 29 23:58:37 UTC 2017

Having some trouble getting DTMF to work where an INVITE comes into
FreeSWITCH offering a=rtpmap:101 telephone-event/16000. FreeSWITCH responds
with a=rtpmap:96 telephone-event/8000 but then is unable to change the
payload type to 96.

This invite is a bit out of the ordinary as it does not specify


*m=audio 63244 RTP/AVP 9 0 101*
*a=rtpmap:101 telephone-event/16000*
*a=fmtp:101 0-15*

//fs logs after INVITE
*2017-06-29 19:28:47.475013 [DEBUG] switch_core_media.c:4192 Set
telephone-event payload to 101 at 16000*
*2017-06-29 19:28:47.475013 [DEBUG] switch_core_media.c:4596
sofia/external/1231231234 at X.X.X.X Set 2833 dtmf send payload to 101 recv
payload to 101*

FreeSWITCH then replies with the following SDP.

*m=audio 33734 RTP/AVP 0 96*
*a=rtpmap:0 PCMU/8000*
*a=rtpmap:96 telephone-event/8000*
*a=fmtp:96 0-16*
*a=silenceSupp:off - - - -*

When the digits come in to FS they are type 96, confirmed via tcpdump
however FreeSWITCH doesn't recognize them and this entry shows up in the
logs when digits are entered.

*2017-06-29 19:29:02.975006 [WARNING] switch_core_media.c:2563 Could not
change to payload type 96, ignoring...*
*2017-06-29 19:29:02.975006 [DEBUG] switch_core_media.c:2540 alternate
payload received (received 96, expecting 0)*

Sofia.conf.xml and external.xml are vanilla. Have tested liberal-dtmf=true
but no luck. Tested on version 1.6.10 and 1.6.18. Was hoping someone might
have a setting to try or any suggestions?

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